1. Field of the Invention
The present invention relates to a signal processing device for detecting a weak signal and a noise strength determining method for a noise to be applied to cause stochastic resonance.
2. Description of the Related Art
Recently, there has been studied a method using stochastic resonance to detect a weak signal including a noise. For example, there has been proposed a parallel-type stochastic resonance circuit in which an input signal containing a weak signal is divided into a plurality of transmission paths and input signals propagating in the respective transmission channels are added with uncorrelated noises (for example, see J. J. Collins, Carson C. Chow, Thomas T. Imhoff, “Stochastic resonance without tuning”, NATURE, Jul. 20, 1995, Vol. 376, pp. 236-238). In this parallel-type stochastic resonance circuit, a plurality of noise-added signals which are added with uncorrelated noises are subjected to threshold processing in parallel and converted into binary signals and these binary signals are combined to reproduce a weak signal. It is known that in this parallel-type stochastic resonance circuit, the accuracy of reproducing a weak signal improves as the number of multiplexed weak signals added with uncorrelated noises increases.
In order to generate a large number of uncorrelated noises, there typically need to be a large number of independent noise sources. To this end, there has been proposed a stochastic resonance circuit equipped with a reduced number of noise sources (for example, see Japanese Patent Application Publication No. 2013-135244, referred to as “JP2013-135244A” below). This stochastic resonance circuit adopts a system using a noise originally contained in an input signal to cause stochastic resonance in a weak signal. Therefore, there is no need to add noise forcibly and there is the advantage of being able to reduce noise sources that generate an uncorrelated noise.
However, in the above-mentioned parallel-type stochastic resonance circuit, in order to generate a plurality of uncorrelated noises, a plurality of noise sources need to be provided, which causes problems of complicating the circuit configuration and increasing the circuit size. In addition, as the stochastic resonance circuit disclosed in JP2013-135244A is configured to cause stochastic resonance using a noise originally included in an input signal, the noise is not always optimal for occurrence of stochastic resonance, which may bring about deterioration of an SN ratio.